How do i delete faces and keep faces next to deleted faces smooth?
You may try adding the extra loopcuts at the edge. Select the whole mesh with A and get rid of tris with Alt+J. Add two loopcuts (Ctrl+R) and place them very close to the edge. Then triangulate the mesh with Ctrl+T. 


Result:



If you don't want to add extra geometry as above just check the Autosmooth checkbox in a Mesh Data header.